I've heard rain is considered a symbol of blessing, perhaps because of the flood: that Hashem doesn't repeat the flood so He sends rain in much smaller proportions and to give the land its nourishment. But what's the source for this idea? Also, is there any symbol in fog?

(Artscroll) translation: It will be that if you hearken to My commandments that I command you today, to love Hashem, your God, and to serve Him with all your heart and with all your soul, then I shall provide rain for your Land in its proper time,the early and the late rains, that you may gather in your grain, your wine, and your oil.